Telecommunications systems are generally structured using an architecture that relies on the open source interconnection (OSI) communication model as standardized by the International Standards Organization (ISO).
The OSI communication model defines management of a data transmission system by using a stack of seven protocol layers: the physical layer (layer 1), the data link layer (layer 2), the network layer (layer 3), the transport layer (layer 4), the session layer (layer 5), the presentation layer (layer 6), and the application layer (layer 7).
The first three layers 1, 2, and 3, referred to as “low” layers, relate to implementing the connection and transporting data. The following four layers, referred to as “high” layers, are responsible for data processing. This organization thus enables the telecommunications system to implement the service associated with the data being processed.
Data link protocols satisfy service requests coming from the network layer and they perform this function by emitting service requests to the physical layer.
Exchanges of signaling between two communication entities separated by a transmission channel are controlled at data link layer level by means of a frame structure, where the frames are said to be MAC frames. MAC frames are encapsulated in a frame structure, referred to as physical frames, by the physical layer PHY prior to being transmitted over the transmission channel. Frames include data frames. The remaining frames are referred to below as control frames.
The data frames under consideration have a generic format as shown in FIG. 1. This format comprises a header, payload data, and a tail field. In this format, each layer encapsulates service data unit (SDU) data coming from the higher layer in one or more protocol data units (PDUs).
Control frames include at least a header.
The exchange of frames between the physical (PHY) and MAC layers of an emitter EM and a receiver RE, is shown by the diagram of FIG. 2. The correspondence between the MAC frames of the two MAC layers is performed via a virtual channel CanalVirt. The physical channel CanalPhy between the PHY layers corresponds to the transmission channel.
The data frame transmitted over the communication channel by the PHY layer is then referred to as the physical layer convergence protocol PDU (PPDU) and the payload data transported by a PPDU is referred to as PSDU. The header of a PPDU contains fields for providing synchronization between the emitter and the receiver, for estimating the transmission channel and containing fields with information needed for receiving the PSDU. The payload data coming from the MAC layer is encapsulated in one or more PPDUs.
The transmission time ttra of a PPDU data frame depends on the transmission mode used and must necessarily be shorter than the access time tacc associated with the access mode. The access time is the time during which the transmission channel is available.
In order to comply with a mean physical data rate constraint Dmoy generally associated with a quality of service QoS that is guaranteed for a given service, the payload data Payload must be transmitted within a duration t that is less than the ratio Payload/Dmoy.
The purpose of link adaptation methods is to ensure a mean data rate Dmoy for an emitter-receiver distance d that takes account of the transmission medium in such a manner as to guarantee a quality of service QoS.
For this purpose, the link adaptation method acts in real time to select a transmission interface and a transmission mode that comply with the constraints of the service (d, data rate, QoS).
Selection thus seeks to obtain the best transmission mode for guaranteeing a data rate and a QoS at a distance d.
A link adaptation algorithm referred to as MiSer is known from [1]. During an initialization step, the method, while not in operation, determines quality tables. For this purpose, the MiSer algorithm considers the following six parameters:
R: the instantaneous data rate of the transmission mode selected for a given interface of WiFi type;
PT: the transmission power of the emitter;
l: the size of the data field for transmission corresponding to a PSDU unit;
s: propagation losses between the emitter and the receiver;
SRC: the number of request-to-emit (RTS) transmission attempts that correspond to a request to access the channel; and
LRC: the number of attempts at transmitting the PPDU.
During initialization, the method estimates two parameters:
L(R,PT,l,s,SRC,LRC): the number of bits that are transmitted correctly; and
E(R,PT,l,s,SRC,LRC): the energy needed to transmit L.
These two estimates are calculated recursively by considering all possible combinations of the parameters R, PT, l, s, SRC, and LRC. Thereafter, on the basis of the two estimated parameters, the method estimates energy efficiency J:
                              J          ⁡                      (                          R              ,                              P                T                            ,              l              ,              s              ,              SRC              ,              LRC                        )                          =                              L            ⁡                          (                              R                ,                                  P                  T                                ,                l                ,                s                ,                SRC                ,                LRC                            )                                            E            ⁡                          (                              R                ,                                  P                  T                                ,                l                ,                s                ,                SRC                ,                LRC                            )                                                          (        1        )            
For each possible combination (l,s,SRC,LRC), the combination (R,PT) that minimizes J is stored.
While the method is running, i.e. not during initialization, the station seeking to transmit estimates the propagation losses s and recovers the number of RTS transmission attempts (SRC) and the number of PPDU transmission attempts (LRC). As a function of the data quadruplet (l,s,SRC,LRC), the station recovers from the quality tables the combination of parameters (R,PT) to be used in order to maximize the energy efficiency of the transmission. That algorithm thus serves to maximize energy efficiency for transmitting a data load, but it is constraining since it requires RTS/CTS frames to be used and it requires an initialization stage while it is not in operation. These frames increase the consumption of the system since emitting RTS/CTS frames has an energy cost. The initialization stage while not operating is necessary in order to calculate the energy efficiency corresponding to all possible combinations of the parameters (R,PT,l,s,SRC,LRC).